In the oil, gas, petroleum, and power industries, emergency shutdown of a process must be provided for under certain fault conditions.
An emergency shutdown (ESD) system is usually implemented by pneumatically controlled shut off valves, which generally remain open while the process is operating safely. These valves are usually only closed when an emergency shut down is required, or for maintenance. Often, processes operate for long periods of time, e.g., years, without shutting down. As the shutdown valves are operated infrequently, there is a high possibility that they will stick or freeze when the shutdown operation is required, thus resulting in a dangerous condition if an emergency shutdown has been requested.
The problem can be exacerbated by economic conditions which lead to a reduction in the frequency of shutdowns or turn-arounds. For example, in some operations a process may run continuously for one or more years without shutting down the process for maintenance.
State-of-the-art ESD systems, which control shut-off valves, have a number of features to detect plant or process failures and typically include redundancies for added reliability. However, such systems may not provide for the testing of shut-off valves themselves, other than full stroking the valve. However, the problem with full stroking or completely closing the valve is that it causes an undesirable disruption in the process. To alleviate the problem, partial stroke testing systems have been developed. In a partial stroke test (PST), a valve is partially closed in order to confirm that it is not stuck in an open position.
PST is not only applicable to safety related applications but can also be used to enhance the operation of the valve. For example, in many process applications, the chemical composition of the flowing stream can cause material to build up on the valve internal body and trim surfaces. Over time this build-up may cause the valve to “stick” in that position and not stroke. PST can be used to simply “exercise” the valve while allowing it to partially stroke, keeping the valve surfaces that are required to move free from material build up.
Many PST systems use mechanical hard stop devices which normally require a purpose-built actuator with integral manually engaged travel stops or add-on type manually engaged stops mounted as an interface between the actuator and the valve. These mechanical stops offer the benefits of hard travel stops to prevent spurious over travel and allow full actuator torque output to operate a valve experiencing stiction. However, they suffer from several disadvantages in that they require extensive operator training and procedures both for engagement and disengagement operations. Furthermore, they typically cannot be immediately disengaged should an ESD occur during partial stroke testing. This severely compromises safety.
Other common PST systems have no hard stops but rely instead on the careful release of air pressure to allow the spring inside of the actuator to move the actuator and valve to a desired partial stroke position which is chosen to both provide maximum valve motion without disrupting the controlled process. However, as only a small percentage of the air pressure can be released, the available torque or force output from the actuator is only a very small percentage of the actuator rated torque or force. As a result, a small change in the valve resistance to motion is sufficient to prevent the small actuator output to cause valve motion. In this situation, additional air must be released to develop sufficient actuator output, however, at the resulting pressure the actuator will cause excessive (spurious) valve travel and a resultant process disruption.
In order to prevent spurious motion, designers of such PST systems program pressure and time limits so that if either is exceeded the PS test is aborted. Thus the user has what is called a false failure whereby maintenance must be performed to determine the cause of the failure. Too often nothing is found other than a slight resistance to valve motion. As the process must be shut down for this maintenance action, the PST system causes the very process disruption that it was designed to prevent.
Fluid driven actuators are selected based on many factors with the most critical being that the actuator provide sufficient torque or thrust to operate an attached valve. It is common, when sizing an actuator for an application, to apply a safety factor to the expected valve requirements to ensure adequate output from the actuator. Typical safety factors range from 1.25 to 2.0 which results in the selection of an actuator having 1.25 to 2.0 times the expected valve torque or thrust requirement. When selecting the safety factor, care is required to assure that the actuator output will not exceed the valve MAST value provided by the valve maker.
Despite the initial design safety factor, once the assembly is installed and operational, users desire to know the actual safety margin at time of installation and also as the assembly ages, where debris accumulation, corrosion and user process changes may impact actuator output and valve requirements.
Some valves are in almost constant motion as they control the flow of fluid. Others see little motion as they are used to isolate a portion of a process to enable maintenance and others have long periods of inactivity as they are employed to safety shutdown a process in the event of an emergency. Users are interested in the installed safety margin for many of their valves but especially for those that are inactive for long periods and which are critical to the safety of the plant and personnel.
As described above, for critical safety shutdown valves, suppliers have offered what is referred to as Partial Stroke Testing Devices, described herein. These are used to exhaust pressure from a spring return actuator until the spring force causes the valve to move to a partially closed position that is less than what would cause disruption of the process. Users are required to periodically perform a partial stroke test to confirm the functional capability of these valves.
The amount of pressure to be exhausted and the time required to move to the partial stroke position is determined by pre-installation testing of the actuator. However, once installed, the valve and the process itself, cause changes to the valve input requirements, and thereby rendering the pre-installation parameters invalid. Establishment of the pressure and time parameters after installation is not practical as the required trials would result is severe process disruption. Thus, unable to test in the active process, and unable to account for changes due to debris, corrosion and process changes, suppliers are forced to program their devices to abort a partial stroke test if the pressure falls below the tested value or if the partial stroke position is not achieved within the tested time. Given the installed pressure and time inherently deviate from the programmed values, users experience considerable numbers of aborted tests, referred to as false failures. The resulting maintenance requirements has led to a diminished interest in partial stroke testing.
Any system designed to monitor actuator and valve performance must do so in a manner that is safe, that does not disrupt the controlled process, and which does not provide false failures.
From the above it can be seen that several parameters are important to the use of the system of the present invention, to wit:                Safety Factor: the ratio of the expected actuator output to the expected valve torque or thrust requirement and is intended to assure that the actuator will properly operate the valve once installed in an active pipeline. Basically, safety factor is a given multiple of the valves' operating torque (a known parameter).        Safety Margin: the actual ratio of actuator output to the actual valve torque or thrust requirement when operating in an active pipeline.        MAST: the Maximum Allowed Stem Torque or Thrust, which is the value at which some part of the valve, usually the stem (or shaft), may be damaged by excessive actuator output.        